The present invention relates generally to intraocular lenses and, more particularly, to a posterior chamber lens for implantation following extracapsular cataract extraction.
For various reasons, the natural lens of a human eye can sometimes become clouded and obstruct the passage of light. This clouding of the lens is known as a cataract and generally worsens with the passage of time. Often times, the cataract becomes so severe that the passage of light through the lens is completely blocked, and the person afflicted with this condition eventually is unable to see. In such instances, it is usually necessary to surgically remove the cataract and replace the natural lens with an artificial intraocular lens to restore vision.
To remove the cataract, a form of surgery known as extracapsular cataract extraction commonly is used. This surgery involves making an incision in the front wall of the eye into the anterior chamber. The pupil is dilated to give access to the posterior chamber where the natural lens is situated. The surgeon then opens the anterior capsule or front wall of the natural human lens and removes the clouded lens nucleus by various known techniques, such as expression or phacoemulsification. The transparent posterior capsule or rear wall of the lens is left intact and, depending upon one's surgical techniques, some peripheral portions of the anterior capsule known as the annular capsular flaps also are left intact. An intraocular lens may then be implanted in the posterior chamber to replace the natural lens and restore vision.
Extracapsular surgery generally has been preferred over other surgical techniques for cataract removal primarily because the intact posterior capsule serves several beneficial functions. For example, the posterior capsule acts as a barrier to the migration of vitreous humor from its normal position behind the lens. The posterior capsule also reduces the chance of developing a condition known as cystoid macula edema, which is a swelling of the macula of the retina. This condition occurs when certain enzymes are released from the iris and migrate through the vitreous humor back to the macula to cause undesirable swelling. Leaving the posterior capsule intact substantially prevents these problems.
While the extracapsular form of surgery has many advantages, there are some problems associated with it. For example, after the clouded lens material is removed, the surgeon typically will scrape or "polish" the posterior capsule to remove as much of the lens material as possible. However, it is virtually impossible to completely remove all the living lens cells from the capsule, and, as time goes by, the remaining cells continue to reproduce and grow, forming a glistening, bubbly, vision-impairing material known as Elschnig's pearls. Another vision-impairing phenemenon, known as fibrosis, occurs when remnant lens fibers on the posterior capsule create opacities which block the passage of light to the retina and cause partial or complete opacification of the capsule. Other potential problems include pigmentary or inflamatory deposits on the posterior capsule, and wrinkling or corrugating of the capsule.
When vision impairment from Elschnig's pearls, fibrosis or other causes is present, it is usually necessary to perform a surgical procedure to open or remove part of the clouded posterior capsule to restore proper vision. The surgical procedure is commonly referred to as a discission or secondary capsulotomy. For many years, discissions have been performed with a laser, such as a neodymium-YAG laser. Laser discissions are now widely used and accepted, primarily because they are relatively safe and effective, and, importantly, they do not require an invasive surgical procedure to open the capsule. The former technique of opening or removing part of the clouded posterior capsule, which has been substantially displaced by the laser technique, involves making an incision in the eye and inserting a needle or other mechanical instrument to tear open the clouded capsule. The primary advantages of the non-invasive nature of the laser discission are that the risks of infection and other complications are reduced, and the procedure can be performed in a few minutes in a doctor's office, instead of in a hospital operating room.
In performing a laser discission, the laser beam is focused on the clouded posterior capsule and the beam is pulsed to sufficiently open the clouded capsule. One of the problems associated with laser discissions is the possible damage to the implanted lens. After implantation, the rear surface of the intraocular lens usually is in intimate contact with the posterior capsule. When the laser beam is focused and pulsed, an explosion occurs at the focused area creating a "sphere of explosion." If the sphere of explosion encompasses the intraocular lens, the lens can sustain damage in the form of vision-impairing pits or cracks. This undesirable lens damage can occur even when the laser beam is properly focused, due to the intimate contact between the lens and posterior capsule. It is therefore advantageous to provide a spacing between the capsule and the implanted lens to position the rear surface of the lens outside the laser beam's sphere of explosion.
Several intraocular lenses have been designed to provide such spacing and prevent damage to the intraocular lens during laser discissions. The pioneer lens in the field is that shown in U.S. Pat. No. Re. 31,626. This lens has a generally annular lip or ridge projecting rearwardly from the posterior surface of the lens, for contacting the posterior capsule and spacing the surface away from the lens. Another lens, shown in U.S. Pat. No. 4,412,359, has a rear lens surface that is concavely formed to space the central rear surface of the lens away from the posterior capsule. Still another lens, shown in U.S. Pat. No. 4,485,499, has a pair of support loops or risers formed separately from the lens, with each riser attached at its extremities to the posterior surface of the lens to space the lens from the posterior capsule.
The present invention is intended to enable another approach for spacing the rear surface of the lens from the posterior capsule which is particularly suited to low cost manufacture and reduced weight.